1. Field of the Invention
The present invention relates to a composite member and separating method therefor, a bonded substrate stack and separating method therefor, a transfer method for transfer layer, and an SOI substrate manufacturing method.
2. Description of the Related Art
A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.
(1) The integration degree can be increased because dielectric isolation is easy.
(2) The radiation resistance can be increased.
(3) The operating speed of the device can be increased because the stray capacitance is small.
(4) No well step is necessary.
(5) Latch-up can be prevented.
(6) A complete depletion type field effect transistor can be formed by thin film formation.
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As one SOI technology, the SOS (Silicon On Sapphire) technology by which Si is heteroepitaxially grown on a single-crystal sapphire substrate by CVD (Chemical Vapor Deposition) has been known for a long time. This SOS technology once earned a reputation as the most matured SOI technology. However, the SOS technology has not been put into practical use to date because, e.g., a large amount of crystal defects are produced by lattice mismatch in the interface between the Si layer and the underlying sapphire substrate, aluminum that forms the sapphire substrate mixes in the Si layer, the substrate is expensive, and it is difficult to obtain a large area.
Attempts have recently been made to realize the SOI structure without using any sapphire substrate. The attempts are roughly classified into two methods.
In the first method, the surface of a single-crystal Si substrate is oxidized, and a window is formed in the oxide film (SiO2 layer) to partially expose the Si substrate. Single-crystal Si is epitaxially grown laterally using the exposed portion as a seed, thereby forming a single-crystal Si layer on SiO2 (in this method, an Si layer is deposited on an SiO2 layer).
In the second method, a single-crystal Si substrate itself is used as an active layer, and an SiO2 layer is formed on the lower surface of the substrate (in this method, no Si layer is deposited).
As a means for realizing the first method, a method of directly epitaxially growing single-crystal Si in the horizontal direction from the single-crystal Si layer by CVD (CVD), a method of depositing amorphous Si and epitaxially growing single-crystal Si laterally in the solid phase by annealing (solid phase epitaxial growth), a method of irradiating an amorphous silicon layer or a polysilicon layer with a focused energy beam such as an electron beam or laser beam to grow a single-crystal Si layer on an SiO2 layer by melting recrystallization (beam annealing), or a method of scanning band-shaped melting regions by a rod-like heater (zone melting recrystallization) is known.
All of these methods have both advantages and disadvantages and many problems of controllability, productivity, uniformity, and quality, and therefore have not been put into practical use in terms of industrial applications. For example, CVD requires sacrifice oxidation to form a flat thin film. Solid phase epitaxial growth is poor in crystallinity. In beam annealing, the process time required to scan the focused beam and controllability for beam superposition or focal point adjustment pose problems. Zone melting recrystallization is the most matured technique, and relatively large-scaled integrated circuits have been fabricated on a trial basis. However, since a number of crystal defects such as a subboundary undesirably remain, minority carrier devices cannot be created.
As the above second method, i.e., as the method without using the Si substrate as a seed for epitaxial growth, the following four techniques can be used.
As the first technique, an oxide film is formed on a single-crystal Si substrate having a V-shaped groove formed in the surface by anisotropic etching. A polysilicon layer having nearly the same thickness as that of the single-crystal Si substrate is deposited on the oxide film. After this, the single-crystal Si substrate is polished from the lower surface, thereby forming, on the thick polysilicon layer, a substrate having a single-crystal Si region surrounded and dielectrically isolated by the V-shaped groove. With this technique, a substrate having satisfactory crystallinity can be formed. However, there are problems of controllability and productivity in association with the process of depositing polysilicon as thick as several hundred micron or the process of polishing the single-crystal Si substrate from the lower surface to leave the isolated Si active layer.
The second technique is SIMOX (Separation by Ion Implanted Oxygen). In this technique, oxygen ions are implanted into a single-crystal Si substrate to form an SiO2 layer. In this technique, to form an SiO2 layer in a substrate, oxygen ions must be implanted at a dose of 1018 (ions/cm2) or more. This implantation takes a long time to result in low productivity and high manufacturing cost. In addition, since a number of crystal defects are generated, the quality is too low to manufacture minority carrier devices.
As the third technique, an SOI structure is formed by dielectric isolation by oxidizing a porous Si layer. In this technique, an n-type Si island is formed on the surface of a p-type single-crystal Si substrate by proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or epitaxial growth and patterning. This substrate is anodized in an HF solution to convert only the p-type Si substrate around the n-type Si island into a porous structure. After this, the n-type Si island is dielectrically isolated by accelerated oxidation. In this technique, since the Si region to be isolated must be determined before the device process, the degree of freedom in device design is limited.
As the fourth technique, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this technique, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, in thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
Selective etching is effective to uniformly thin the substrate. However, the selectivity ratio is as low as about 102, the surface planarity after etching is poor, and the crystallinity of the SOI layer is unsatisfactory.
A transparent substrate represented by a glass substrate is important in forming a contact sensor as a light-receiving element or a projection liquid crystal display device. To realize highly precise pixels (picture elements) having higher density and resolution for the sensor or display device, a high-performance driving element is required. For this purpose, a demand has arisen for a technique of forming a single-crystal Si layer having excellent crystallinity on a transparent substrate.
However, when an Si layer is deposited on a transparent substrate represented by a glass substrate, only an amorphous Si layer or a polysilicon layer is obtained. This is because the transparent substrate has an amorphous crystal structure, and the Si layer formed on the substrate reflects the disorderliness of the crystal structure of the transparent substrate.
The present applicant has disclosed a new SOI technology in Japanese Patent Laid-Open No. 5-21338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred-xc3x85 to 10-xcexcm thick SOI films can be manufactured by a single manufacturing apparatus.
The present applicant has also disclosed, in Japanese Patent Laid-Open No. 7-302889, a technique of bonding first and second substrates, separating the first substrate from the second substrate without breaking the first substrate, smoothing the surface of the separated first substrate, forming a porous layer again on the first substrate, and reusing this substrate. Since the first substrate is not wasted, this technique is advantageous in largely reducing the manufacturing cost and simplifying the manufacturing process.
To separate the bonded substrate stack into two substrates without breaking the first and second substrates, for example, the two substrates are pulled in opposite directions while applying a force in a direction perpendicular to the bonding interface, a shearing force is applied parallel to the bonding interface (for example, the two substrates are moved in opposite directions in a plane parallel to the bonding interface, or the two substrates are rotated in opposite directions while applying a force in the circumferential direction), a pressure is applied in a direction perpendicular to the bonding interface, a wave energy such as an ultrasonic wave is applied to the separation region, a peeling member (e.g., a sharp blade such as knife) is inserted into the separation region parallel to the bonding interface from the side surface of the bonded substrate stack, the expansion energy of a substance filling the pores of the porous layer functioning as the separation region is used, the porous layer functioning as the separation region is thermally oxidized from the side surface of the bonded substrate stack to expand the volume of the porous layer and separate the substrates, or the porous layer functioning as the separation region is selectively etched from the side surface of the bonded substrate stack to separate the substrate.
Porous Si was found in 1956 by Uhlir et al. who were studying electropolishing of semiconductors (A. Uhlir, Bell Syst. Tech. J., vol. 35, 333 (1956)). Porous Si can be formed by anodizing an Si substrate in an HF solution.
Unagami et al. studied the dissolution reaction of Si upon anodizing and reported that holes were necessary for anodizing reaction of Si in an HF solution, and the reaction was as follows (T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)).
Si+2HF+(2xe2x88x92n)e+xe2x86x92SiF2+2H+nexe2x88x92
SiF2+2HFxe2x86x92SiF4+H2
SiF4+2HFxe2x86x92H2SiF6
or
Si+4HF+(4xe2x88x92xcex)e+xe2x86x92SiF4+4H+xcexexe2x88x92
SiF4+2HFxe2x86x92H2SiF6
where e+ and exe2x88x92 represent a hole and an electron, respectively, and n and xcex are the number of holes necessary to dissolve one Si atom. According to them, when n greater than 2 or xcex greater than 4, porous Si is formed.
The above fact suggests that p-type Si having holes is converted into porous Si while n-type Si is not converted. The selectivity in this conversion has been reported by Nagano et al. and Imai (Nagano, Nakajima, Anno, Onaka, and Kajiwara, IEICE Technical Report, vol. 79, SSD79-9549 (1979)), (K. Imai, Solid-State Electronics, vol. 24, 159 (1981)).
However, it has also been reported that n-type at a high concentration is converted into porous Si (R. P. Holmstrom and J. Y. Chi, Appl. Phys. Lett., vol. 42, 386 (1983)). Hence, it is important to select a substrate which can be converted into a porous Si substrate independently of p- or n-type.
To form a porous layer, in addition to anodization, ions may be implanted into a silicon substrate.
For example, in the method described in Japanese Patent Laid-Open No. 5-21338, i.e., the method in which a substrate (to be referred to as a bonded substrate stack hereinafter) obtained by bonding a first substrate having a non-porous layer such as a single-crystal Si layer on a porous layer to a second substrate via an insulating layer is separated at the porous layer, thereby transferring the non-porous layer formed on the first substrate side to the second substrate, the technique of separating the bonded substrate stack is very important.
For example, in separating the bonded substrate stack, if it is separated at a portion except the porous layer as the separation layer, the non-porous layer (e.g., a single-crystal Si layer) to be used as an active layer is broken, and no desired SOI substrate can be obtained.
The present invention has been made in consideration of the above situation, and has as its object to make it possible to appropriately separate a composite member such as a bonded substrate stack at a separation layer such as a porous layer.
According to the first aspect of the present invention, there is provided a method of separating a composite member having a structure in which a first member having a separation layer inside is brought into tight contact with a second member, characterized in that the composite member has a projecting portion at which an outer peripheral edge of the first member projects outside that of the second member, and the method comprises the separation step of starting separating the composite member from the projecting portion and then separating the composite member into two members at the separation layer.
In the composite member separating method according to the first aspect of the present invention, preferably, for example, a major surface of the first member has the same shape as that of the second member, and the composite member has a structure in which the major surface of the first member is brought into tight contact with the major surface of the second member while shifting central positions from each other.
In the composite member separating method according to the first aspect of the present invention, preferably, for example, a major surface of the second member is smaller than that of the first member, and the composite member has a structure in which the major surface of the first member is brought into tight contact with the major surface of the second member.
In the composite member separating method according to the first aspect of the present, invention, for example, the separation step preferably comprises the pre-separation step of processing the projecting portion to form a separation start portion, and the main separation step of starting separating the composite member from the separation start portion and then breaking substantially only the separation layer to separate the composite member into two members at the separation layer.
According to the second aspect of the present invention, there is provided a composite member manufacturing method characterized by comprising bringing a first member having a separation layer inside into tight contact with a second member to manufacture a composite member having a projecting portion at which an outer peripheral edge of the first member projects outside that of the second member.
According to the third aspect of the present invention, there is provided a method of transferring a transfer layer on a surface of a first member to a second member, characterized by comprising the preparation step of bringing a first member having a separation layer inside and the transfer layer on the separation layer into tight contact with a second member to prepare a composite member in which an outer peripheral edge of the first member projects outside that of the second member, and the separation step for starting separating the composite member from the projecting portion and separating the composite member into two members at the separation layer, thereby transferring the transfer layer of the first member to the second member.
According to the fourth aspect of the present invention, there is provided a separating method of separating into two substrates a bonded substrate stack having a structure formed by bringing a transfer layer of a first substrate having a separation layer inside and the transfer layer on the separation layer into tight contact with a second substrate, characterized in that the bonded substrate stack has a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate, and the separation method comprises the separation step of starting separating the bonded substrate stack from the projecting portion and then separating the bonded substrate stack into two substrates at the separation layer.
In the bonded substrate stack separating method according to the fourth aspect of the present invention, preferably, for example, the first substrate has the same size as that of the second substrate, and the bonded substrate stack has a structure in which the first substrate and the second substrate are brought into tight contact while shifting central positions from each other.
In the bonded substrate stack separating method according to the fifth aspect of the present invention, for example, the bonded substrate stack preferably has a structure in which the first substrate is brought into tight contact with the second substrate smaller than the first substrate.
In the bonded substrate stack separating method according to the fifth aspect of the present invention, preferably, for example, the second substrate has one of an orientation flat and a notch, and the bonded substrate stack has, as the projecting portion, a portion where the first substrate is exposed in the presence of the orientation flat or notch of the second substrate.
In the bonded substrate stack separating method according to the fifth aspect of the present invention, preferably, for example, each of the first substrate and the second substrate has one of an orientation flat and a notch, and the bonded substrate stack is formed by bringing the first substrate and the second substrate into tight contact with each other without matching the orientation flat or notch of the first substrate with the orientation flat or notch of the second substrate, and has, as the projecting portion, a portion where the first substrate is exposed in the presence of the orientation flat or notch of the second substrate.
According to the sixth aspect of the present invention, there is provided a bonded substrate stack manufacturing method characterized by comprising bonding a transfer layer of a first substrate having a separation layer inside and the transfer layer on the separation layer to a second substrate to manufacture a bonded substrate stack having a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate.
According to the seventh aspect of the present invention, there is provided a transfer method of transferring a transfer layer on a surface of a first substrate to a second substrate, characterized by comprising the preparation step of bonding the transfer layer of the first substrate having a separation layer inside and the transfer layer on the separation layer to the second substrate to prepare a bonded substrate stack having a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate, and the separation step of starting separating the bonded substrate stack from the projecting portion and then separating the bonded substrate stack at the separation layer, thereby transferring the transfer layer of the first substrate to the second substrate.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate and the second substrate, which have the same size, into tight contact with,each other while shifting central positions to prepare the bonded substrate stack.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate into tight contact with the second substrate smaller than the first substrate to prepare the bonded substrate stack.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate into tight contact with the second substrate having one of an orientation flat and a notch to prepare the bonded substrate stack having, as the projecting portion, a portion where the first substrate is exposed in the presence of the orientation flat or notch of the second substrate.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the preparation step preferably comprises preparing the first substrate and the second substrate each having one of an orientation flat and a notch, and bringing the first substrate and the second substrate into tight contact with each other without matching the orientation flat or notch of the first substrate with the orientation flat or notch of the second substrate to prepare the bonded substrate stack.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation step preferably comprises the pre-separation step of processing the projecting portion to form a separation start portion and starting separating the bonded substrate stack from the separation start portion and then breaking substantially only the separation layer to separate the bonded substrate stack into two substrates at the separation layer.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the pre-separation step preferably comprises ejecting a fluid to the projecting portion to form the separation start portion by the fluid.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the pre-separation step preferably comprises inserting a wedge-shaped member into a gap between the first substrate and the second substrate at the projecting portion to form the separation start portion.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the pre-separation step preferably comprises supplying a vibration energy to the projecting portion to form the separation start portion.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the pre-separation step preferably comprises dipping the projecting portion in a liquid and supplying a vibration energy to the projecting portion through the liquid to form the separation start portion.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, water is preferably used as the liquid.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, an etchant is preferably used as the liquid.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the pre-separation step preferably comprises etching the transfer layer and the separation layer at the projecting portion to form the separation start portion.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation step preferably comprises ejecting a fluid to the projecting portion to form the separation start portion on the bonded substrate stack and continuing separating the bonded substrate stack while changing a position where the fluid is injected.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation step preferably comprises inserting a wedge-shaped member into a gap between the first substrate and the second substrate at the projecting portion to separate the bonded substrate stack.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation start portion is preferably a portion where the separation layer at the portion has the most fragile structure.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation start portion is preferably a portion where the transfer layer at the portion is removed, and the separation layer under the transfer layer is exposed.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation start portion is preferably a portion where the separation layer at the portion is exposed, and an outer peripheral edge of the separation layer is located inside the bonded substrate stack.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the separation layer is preferably a porous layer.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the first substrate is preferably a substrate formed by anodizing a substrate to form a porous layer as the separation layer and forming the transfer layer on the separation layer.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the first substrate preferably has, as the separation layer, a porous layer formed by ion implantation.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the transfer layer preferably includes a single-crystal Si layer.
In the transfer layer transfer method according to the seventh aspect of the present invention, for example, the transfer layer preferably sequentially has a single-crystal Si layer and an insulating layer as the transfer layer.
According to the eighth aspect of the present invention, there is provided a method of manufacturing an SOI substrate, characterized by comprising the preparation step of bringing a surface of a first substrate having a porous layer inside and a transfer layer including a single-crystal Si layer on the porous layer into tight contact with a second substrate to prepare a bonded substrate stack having a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate, the separation step for starting separating the bonded substrate stack from the projecting portion and separating the bonded substrate stack at the porous layer, thereby transferring the transfer layer of the first substrate to the second substrate, and the removal step of removing the porous layer remaining on a surface of the second substrate after separation.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the first substrate preferably has, as the transfer layer, not only the single-crystal Si layer but also an insulating layer on the single-crystal Si layer.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the second substrate preferably has an insulating layer on a surface.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate and the second substrate, which have the same size, into tight contact with each other while shifting central positions to prepare the bonded substrate stack.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate into tight contact with the second substrate smaller than the first substrate to prepare the bonded substrate stack.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate into tight contact with the second substrate while making the central positions match with each other.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the preparation step preferably comprises bringing the first substrate into tight contact with the second substrate having one of an orientation flat and a notch to prepare the bonded substrate stack having, as the projecting portion, a portion where the first substrate is exposed in the presence of the orientation flat or notch of the second substrate.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the preparation step preferably comprises preparing the first substrate and the second substrate each having one of an orientation flat and a notch, and bringing the first substrate and the second substrate into tight contact with each other without matching the orientation flat or notch of the first substrate with the orientation flat or notch of the second substrate to prepare the bonded substrate stack.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation step preferably comprises the pre-separation step of processing the projecting portion to form a separation start portion, and starting separating the bonded substrate stack from the separation start portion and then breaking substantially only the porous layer to separate the bonded substrate stack into two substrates at the porous layer.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the pre-separation step preferably comprises ejecting a fluid to the projecting portion to form the separation start portion by the fluid.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the pre-separation step preferably comprises inserting a wedge-shaped member into a gap between the first substrate and the second substrate at the projecting portion to form the separation start portion.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the pre-separation step preferably comprises supplying a vibration energy to the projecting portion to form the separation start portion.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the pre-separation step preferably comprises dipping the projecting portion in a liquid and supplying a vibration energy to the projecting portion through the liquid to form the separation start portion.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, water is preferably used as the liquid.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, an etchant is preferably used as the liquid.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the pre-separation step preferably comprises etching the transfer layer and the porous layer at the projecting portion to form the separation start portion.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation step preferably comprises ejecting a fluid to the projecting portion to form the separation start portion on the bonded substrate stack and continuing separating the bonded substrate stack while changing a position where the fluid is injected.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation step preferably comprises inserting a wedge-shaped member into a gap between the first substrate and the second substrate at the projecting portion to separate the bonded substrate stack.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation start portion is preferably a portion where the porous layer at the portion has the most fragile structure.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation start portion is preferably a portion where the transfer layer at the portion is removed, and the porous layer under the transfer layer is exposed.
In the SOI substrate manufacturing method according to the eighth aspect of the present invention, for example, the separation start portion is preferably a portion where the porous layer at the portion is exposed, and an outer peripheral edge of the porous layer is located inside the bonded substrate stack.
According to the ninth aspect of the present invention, there is provided a composite member having a structure in which a first member having a separation layer inside is brought into tight contact with a second member, characterized by comprising a projecting portion at which an outer peripheral edge of the first member projects outside that of the second member.
According to the 10th aspect of the present invention, there is provided a bonded substrate stack having a structure in which a first substrate having a separation layer inside and a transfer layer on the separation layer is brought into tight contact with a second substrate, characterized by comprising a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate.
According to the 11th aspect of the present invention, there is provided a bonded substrate stack formed by bonding a surface of a first substrate having a porous layer inside and a transfer layer including a single-crystal Si layer on the porous layer to a second substrate, characterized by comprising a projecting portion at which an outer peripheral edge of the first substrate projects outside that of the second substrate.
Further objects, features and advantages of the present invention will become apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings.